JSON in Elm

NOTE: This package is deprecated in favor of the core library one (Json.Decode, Json.Encode) which it inspired

Convenient and composable translation between JSON and Elm types, with helpful error messages.

The main use cases of this library are:

• Creating composable and extensible JSON codecs.
• Decoding the bits of JSON that you actually care about.

Decoding examples

Creating and composing decoders

import Json.Decoder (..)
import Json.Process (into)

type Person = { name: String, age: Int, profession: String }
type Comment = { msg: String, author: Person }
type BlogPost = { content: String, comments: [Comment] }

-- Design the decoders

person : Decoder Person
person = decode3 ("name" := string) ("age" := int) ("profession" := string) Person

comment : Decoder Comment
comment = decode2 ("msg" := string) ("author" := person) Comment

blogpost : Decoder BlogPost
blogpost = decode2 ("content" := string) ("comments" := listOf comment) BlogPost

-- Example data

testdata1 = "{\"name\":\"Jane\",\"age\":47}"
testdata2 = "{\"content\":\"hello world\",\"comments\":[{\"msg\":\"Hello\",\"author\":{\"name\":\"Jane\",\"age\":37,\"profession\":\"Aerospace Engineering\"}},{\"msg\":\"Hello\",\"author\":{\"name\":\"Tim\",\"age\":37,\"profession\":\"Wizard\"}}]}"
testdata3 = "[true,false]"

print : Decoder a -> String -> Element
print decoder s = fromString s `into` decoder |> asText

main = flow down [ print person  testdata1         -- Error ("Could not decode: \'profession\'")
                 , print blogpost testdata2        -- Success { comments = [{ author = { age = 37, ... } ...}], ... }
                 , print (listOf bool) testdata3 ] -- Success [True,False]

Creating accessors and composing them with decoders

import Json.Accessor (delve)
import Json.Decoder (..)
import Json.Process (fromString, into, glue)

type Person = { name: String, age: Int, profession: String }

person : Decoder Person
person = decode3 ("name" := string) ("age" := int) ("profession" := string) Person

accessPerson : Decoder Person
accessPerson = delve [ "x", "y", "z" ] `glue` person

-- Example data

testdata1 = "{\"x\":{\"y\":{\"z\":{\"name\":\"Alice\",\"age\":85,\"profession\":\"Science\"}}}}"
testdata2 = "{\"x\":{\"y\":{\"z\":42}}}"

print : Decoder a -> String -> Element
print decoder s = fromString s `into` decoder |> asText

main = flow down [ print accessPerson testdata1   -- Success { age = 85, name = "Alice", ... }
                 , print accessPerson testdata2 ] -- Error ("Could not access a \'name\' in \'Number 42\'")

Combining alternative decoders for a type

import Json.Accessor (delve)
import Json.Decoder (..)
import Json.Process (fromString, into, or)

type Person = { name: String, age: Int, profession: String }

person1 : Decoder Person
person1 = decode3 ("name" := string) ("age" := int) ("profession" := string) Person

person2 : Decoder Person
person2 = decode2 ("age" := int) ("profession" := string) (Person "Guest")

person3 : Decoder Person
person3 = decode3 ("nick" := string) ("age" := int) ("profession" := string) Person

-- Combined as alternatives
person4 : Decoder Person
person4 = person1 `or` person2 `or` person3 

testdata1 = "{\"age\":45,\"profession\":\"Surfer\"}"

print : Decoder a -> String -> Element
print decoder s = fromString s `into` decoder |> asText

main = flow down [ print person1 testdata1   -- Error ("Could not access a \'name\' in ...")
                 , print person2 testdata1   -- Success { age = 45, name = "Guest", ... }
                 , print person3 testdata1   -- Error ("Could not access a \'nick\' in ...")
                 , print person4 testdata1 ] -- Success { age = 45, name = "Guest", ... }